Marine geophysical surveying is used to assess amongst other things, the topography of the seabed, the location of mineral deposits such as oil and gas, and more recently to assess the quantity of mineral deposits at a particular location. The most commonly used form of geophysical surveying used employs seismic sources which produce sound waves mechanically using high pressure air devices.
When such seismic surveying first took place a single seismic source and following streamer were towed through a body of water by a towing vessel with the seismic source being activated at intervals, the streamer detecting reflected sound from the seabed. Such a system is known as a 2 dimensional system since the survey is conducted along the path taken by the streamer cable. Such surveys allowed information to be gathered that would not previously have been possible. However, two dimensional systems have significant imitations. For example, if a mineral deposit were found the information gleaned by the 2 dimensional survey would approximate to a slice through the mineral deposit.
In order to improve seismic surveying a three dimensional system was developed. In this system a rig comprising multiple cables, often twelve or more, and supporting multiple seismic sources is towed by a vessel.
In the case of the mineral deposit referred to above, the information gleaned from a three dimensional survey can be considered to approximate to multiple slices through the mineral deposit.
As oil and gas becomes more scarce there is interest in surveying previously exploited oil and gas wells, since amounts of such mineral resources may be present that would flow for profitable extraction.
A new system of geophysical surveying, known as four dimensional or time elapsed surveying has been developed. In four dimensional/time elapsed surveying the fourth dimension is time, that is three dimensional surveys are repeated from time to time. If conducted accurately, the extent to which an oil well is being depleted can be measured. The information gleaned from such surveys allows resources to be directed efficiently.
However, a four dimensional survey is only useful if it is possible to conduct almost exactly the same three dimensional survey multiple times separated by time. To achieve this the seismic sources must be fired when they are in substantially the same location when each survey takes place. This can only be achieved by steering the cable rig.
A number of steerable streamer cable rigs have been proposed.
WO2011116118 describes a system for steering actively a seismic survey array. A streamer rig includes a steering system comprises a capstan having cables extending left and right therefrom so that when the right hand cable is pulled in the left hand cable is paid out and vice versa. One of these cables is attached to a lead-in cable that forms part of the rig and is to one side of the streamer array.
The problem associated with this type of arrangement is that the capstan is part of the rig which is dragged through the water. The capstan must therefore operate in a very harsh environment and is hence liable to breakdown.
GB2436456 describes an alternative arrangement where the cables supporting the seismic sources are connected to a winch or capstan on the towing vessel. Actuation of the winch or capstan changes the length of the streamer cables, which changes the position of the array relative to the towing vessel.
GB2414804 describes another alternative arrangement where the cables supporting the seismic sources are connected to outwardly situated lead in cables and winches or capstans on the towing vessel arranged so that when one cable is pulled in by one of the winches/capstans, the other cable is paid out by the other winch capstan.
Each of the above mentioned rigs steers the streamer cables by changing the length of a cable that forms part of the rig.
It would be desirable to provide an alternative means of steering a rig so as to be able to perform four dimensional surveys.